As application of a radiation measurement device to a device in the nuclear medicine field, there is a single photon emission computed tomography (SPECT) using a gamma camera. The SPECT provides a transaxial image by measuring a distribution of a compound containing a radioactive isotope. The mainstream of the conventional SPECT device is a combination of a scintillator made of a single crystal with multiple photomultiplier tubes. Such SPECT devices obtain the location of radiation by centroid computation. However, in such a method, the finest possible resolution is about 10 mm, which is insufficient for the practical use in clinical activity. Therefore, there has been a demand for SPECT devices having higher resolution.
There has recently been developed a pixelated detector as a device having higher resolution. Examples of the pixelated detector include one formed of a scintillator, one formed of a semiconductor, and the like. Any of such pixelated detectors acquire a position signal per unit of a small detector, i.e., per pixel. Therefore, the intrinsic special resolution of the detector is determined by the pixel size, and spatially discrete measurement is performed. There has been developed a device even having a pixel size of 1.2 mm, which achieves the resolution of 10 mm or less, resulting in significant improvement.
There has also been developed and improved a trans axial reconstruction method, making a significant contribution to the improvement in resolution. A filtered back-projection method (FBP method) and a successive approximation method (MLEM, OSEM or the like) without resolution recovery have heretofore been used. Furthermore, a successive approximation method with resolution recovery has also recently been developed. This method enables reconstruction considering geometric configurations of the collimator and the detector, and physical factors such as scattered radiation. Therefore, more accurate images is provided.
The following description of the pixelated detector is provided by using the terms “detector” and “detector group.” The detector means one included in one pixel having any shape, while the detector group means an assembly of the detectors arrayed. The detector generally has a rectangular shape. When seen from the radiation incident side, the detector group has a configuration in which rectangles are densely packed. In order that all the detectors included in the detector group can have uniform sensitivity, the through-holes in the collimator and the detectors are often arranged on a one-to-one basis.
Moreover, in terms of ease in handling, the through-hole generally has a rectangular shape corresponding to the shape of the detector. When the detector has the rectangular shape, two to four surfaces of each detector come into contact with the adjacent detectors, and these surfaces are defined as “boundary surfaces of the detector”. In the conventional device, the ceptor of the collimator is disposed on the boundary surface.
In this regard, there is a generally known problem that moire is generated when there is a positional displacement between the collimator and the detector. In order to solve this problem, there has been disclosed a configuration in which a collimator and a detector group are rotated relative to each other (Patent Literature 2). In this configuration, even when the collimator is displaced from a predetermined position, the area of the ceptor traversing the detector is maintained constant.
A SPECT imaging device with high spatial resolution and high sensitivity has been demanded in clinical practice. There are many factors that determine the resolution and sensitivity, such as a distance from a radiation source, the thickness of the ceptor, radiation energy, scatter, and absorption. Among these factors, the height of the ceptor and the size of the opening are significantly responsible for the determination of the resolution and sensitivity. In order to achieve high resolution, the arrival direction of radiation entering the detector needs to be limited by the collimator. Therefore, a field of view of the detector on the measurement target may be narrowed by the collimator. As such a collimator, there has been known a LEHR (low energy high resolution) collimator, for example. However, such limitation costs the sensitivity.
In order to achieve high sensitivity, the hole length of the collimator needs to be reduced. As such a collimator, there have been known a LEGP (low energy general purpose) collimator and a LEHS (low energy high sensitivity) collimator. However, the shortened hole length of the collimator deteriorates the resolution.
As described above, the conventional device cannot achieve both the high resolution and high sensitivity. Therefore, the collimators need to be replaced according to the purpose, leading to increased workload at a clinical site.
Therefore, as a device that achieves both sensitivity and resolution, a new type of SPECT device has been invented, including two or more detectors in one rectangular through-hole. This SPECT device has been proven to achieve higher resolution than the conventional device in which the through-holes and the detectors correspond on a one-to-one basis, when the SPECT and conventional devices have the through-holes in the same size (Patent Literature 1 and Non-Patent Literature 1).